A fuel injection arrangement of the kind described above is disclosed in U.S. Pat. No. 4,700,668. The pulse chamber is charged directly with the pressure present in the crankcase whereas the return chamber communicates with the atmosphere. The pump piston is moved up and down in correspondence to changes in pressure in the crankcase and fuel is thereby injected into the combustion chamber of the two-stroke engine.
The pressure in the crankcase is dependent upon the rotational speed and the load of the two-stroke engine. An overpressure develops in the crankcase with a downward movement of the piston in the direction toward bottom dead center whereas the pressure in the crankcase drops to an underpressure with the following upward movement of the piston toward top dead center. In this way, the crankcase pressure changes between positive and negative values with the positive values likewise increasing to a maximum with increasing rotational speed which then remain constant up to the highest speed. The pressure changes lie, for example, approximately between 0.75 bar and -0.2 bar.
If the crankcase pressure is also present directly in the pulse chamber of the injection pump, then the injection operation will begin already when the pressure value "zero" is crossed over. Pressure is built up in the crankcase starting approximately at 60.degree. after top dead center. This leads to the situation that fuel injection begins already when there is still an ongoing combustion from the previous cycle. Furthermore, the injection begins already before the overflow channels are open so that fuel is received almost in the same manner as with the carburetor. However, the purpose of the injection is to first admit fresh air from the overflow channels and then perform the injection.
In order to determine the time of injection in advance, U.S. Pat. No. 4,700,668 suggests providing a control or pilot valve between the pulse chamber and crankcase which clears the pressure-transmitting connection between the crankcase and pulse chamber only after a threshold pressure is exceeded.
A control valve of this kind is, however, complex in its configuration and duration and requires a precise matching and regular maintenance in order to prevent a drift of the response threshold. The point of time of fuel injection is also not precisely determined because of the changes of the pressure in the crankcase with speed.